This invention generally relates to any controllable motor and to systems, such as refrigeration systems, having motors therein for driving compressors.
Original equipment manufacturers of refrigeration systems and the like require motors for driving compressors or other rotatable components. Often, variable speed motors, including electronically commutated motors, are too costly for application in typical refrigeration systems. Manufacturers require motors that are as low cost as possible yet still perform as desired. As an example, presently available motors incorporate integration of the back electromotive force (EMF) voltage in the windings to provide robust and flexible rotor position sensing. However, such motors employ a costly transconductance amplifier or similar circuit resulting in greater cost for compressor drive systems.
The control circuits for these electronically controlled motors support various applications through the use of circuitry external to an application specific or universal control circuit. External support, though, increases the parts count as well as the size of the motors which also results in greater cost. For example, the cost of a conventional motor driving a compressor has an additional cost associated with an external sump heater. Since liquids are generally incompressible, damage can result to the compressor if liquid refrigerant is present in the intake to the compressor when the motor is started. The liquid refrigerant must first be converted to a gas. Again, including a separate heater for vaporizing the refrigerant at cold starts increases the cost of the motor.
Also, in a typical motor, current sensors are required in all phases to determine the currents flowing in the motor. However, this feature increases the cost of the motor. On the other hand, a motor employing only a single current sensing element is generally unable to detect circulating currents. As disclosed in commonly assigned application Ser. No. 08/219,022, the entire disclosure of which is incorporated herein by reference, circulating currents cannot be observed in a shunt resistor (or other current sensor) located in the power supply link connecting the motor to a power supply. For this reason, a motor providing detection of circulating currents with a single sensing element is desired.
A further shortcoming of a shunt resistor (or other current sensor) located in the power supply link is the inability to limit current in all three windings at commutation to a desired regulate value. At commutation, the current of two windings (one just turned off and one just turned on) must sum in the third winding of a three-phase motor. At low speed, these currents can sum to a value greater than that observed in the shunt resistor. Therefore, for motors with low ability to withstand demagnetization, some increased risk of motor demagnetization will result. As is known in the art, inductive current in the windings produces a field collapse voltage in the windings of a motor in advance of back EMF. There is a need for a motor employing this field collapse voltage to provide the necessary timing information for the initiation of conduction and the avoidance of current being developed in the third winding above the desired regulate value.
Another shortcoming of presently available motors is that an original equipment manufacturer may have difficulty adapting a conventional motor to a number of different applications requiring a wide range of speeds. In a motor using digital speed error counting for speed regulation, the rate of counting may be suitable at high motor speeds but cause unstable operation at low speeds. Conversely, if the count rate provides stable operation at low speeds, the motor may have insufficient capability to modulate torque at high speeds.
With respect to regulation of the current in the motor's windings, every motor does not respond in the same manner to having its current turned off for a particular angle of off time. Therefore, there is a need to improve motor performance by selecting an angle for current variable off time particular to the motor's application.
Also, microcomputers used in controlling motors have a propensity to "lock up" in the illegal execution of code due to the interruption of sustained serial communication caused by a high noise environment. Therefore, there is a need to employ a "watch dog" which responds to such a lock up by resetting the microcomputer to a state where correct operation can resume.
Motors, such as electronically commutated motors, are disclosed in U.S. Pat. Nos. 4,642,537, 4,757,241 and 4,806,833 and application Ser. No. 07/889,708, filed May 27, 1992, all of which are commonly assigned with the present application and the entire disclosures of which are incorporated herein by reference in their entirety.